1. Field of the Invention
This invention relates to the field of cables and cable terminations. More specifically, the invention comprises a cable termination including an angularly offset parabolic wall profile.
2. Description of the Related Art
There are many known devices for mounting a termination on the end of a wire, rope, or cable. The individual components of a wire rope are generally referred to as “strands,” whereas the individual components of natural-fiber cables or synthetic cables are generally referred to as “fibers.” For purposes of this application, the term “strands” will be used generically to refer to both.
In order to carry a tensile load an appropriate connective device must be added to a cable. A connective device is typically added to an end of the cable, but may also be added at some intermediate point between the two ends. FIG. 1 shows a connective device which is well known in the art. An anchor 18 has been attached to the free end of a cable 10 to form a termination 14.
FIG. 2 shows the same assembly sectioned in half to show its internal details. Anchor 18 includes internal passage 28 running through its mid portion. In order to affix anchor 18 to cable 10, the strands proximate the end of cable 10 are exposed and placed within internal passage 28 (They may also be splayed or fanned to conform to the expanding shape of the passage).
Liquid potting compound is added to the region of strands lying within the anchor (either before or after the strands are placed within the anchor). This liquid potting compound solidifies while the strands are within the anchor to form potted region 16 as shown in FIG. 2. Most of potted region 16 consists of a composite structure of strands and solidified potting compound. Potting transition 20 is the boundary between the length of strands which is locked within the solidified potting compound and the freely-flexing length within the rest of the cable (flexible region 30).
The unified assembly shown in FIGS. 1 and 2 is referred to as a “termination” (designated as “14” in the view). The mechanical fitting itself is referred to as an “anchor” (designated as “18” in the view). Thus, an anchor is affixed to a cable to form a termination. These terms will be used consistently throughout this disclosure.
Cables such as the one shown in FIG. 2 are used to carry tensile loads. When a tensile load is placed on the cable, this load must be transmitted to the anchor, and then from the anchor to whatever component the cable attaches to (typically through a thread, flange, or other fastening feature found on the anchor). As an example, if the cable is used in a winch, the anchor might include a large hook.
Those skilled in the art will realize that potted region 16 is locked within anchor 18 by a mechanical interference resulting from the geometry of internal passage 28. FIG. 3 is a sectional view showing the potted region removed from the anchor. As shown in FIG. 3, internal passage 28 molds the shape of potted region 16 so that a mechanical interference is created between the two surfaces (which in this case are conical surfaces). When the potted region first solidifies, a surface bond is often created between the potted region and the wall of the tapered cavity. When the cable is initially loaded, the potted region is pulled downward (with respect to the orientation shown in the view) within the tapered cavity. This action is often referred to as “seating” the potted region. The surface bond typically fractures. Potted region 16 is then retained within tapered cavity 28 solely by the mechanical interference of the mating male and female conical surfaces.
FIG. 4 shows the assembly of FIG. 3 in a sectioned elevation view. The wall profile is revolved around central axis 51, which runs through the anchor from neck anchor boundary 48 to distal anchor boundary 50. One can define the slope of the wall profile at any point along the internal passage with respect to this central axis. For purposes of this disclosure, a positive slope for the wall profile will mean a slope in which the distance from the central axis to the wall is increasing as one proceeds from the proximal anchor boundary to the distal anchor boundary.
As mentioned previously, the seating process places considerable shearing stress on the surface bond between the potted region and the wall, which often breaks. Further downward movement is arrested by the compressive forces exerted on the potted region by the shape of the internal passage (Spatial terms such as “downward”, “upper”, and “mid” are used throughout this disclosure. These terms are to be understood with respect to the orientations shown in the views. The assemblies shown can be used in any orientation. Thus, if a cable assembly is used in an inverted position, what was described as the “upper region” herein may be the lowest portion of the assembly).
The compressive stress on potted region 16 tends to be maximized in neck region 22. Flexural stresses tend to be maximized in this region as well, since it is the transition between the freely flexing and rigidly locked regions of the strands. The tensile stresses within potted region 16 likewise tend to be maximized in neck region 22, since it represents the minimum cross-sectional area. Thus, it is typical for terminations such as shown in FIGS. 1-4 to fail within neck region 22.
In FIG. 4, potted region 16 is conceptually divided into neck region 22, mid region 24, and distal region 26. Potting transition 20 denotes the interface between the relatively rigid potted region 16 and the relatively freely flexing flexible region 30. Stress is generally highest in neck region 22, lower in mid region 24, and lowest in distal region 26.
The prior art anchor shown in FIGS. 1-4 uses a revolved linear wall profile (a conical shape for the internal passage). While this profile is commonly used, it is far from optimum. The design considerations present in the neck region, mid region, and distal region are quite different. FIG. 5 illustrates—in very general terms—the nature of these design considerations. In neck region 22, the wall profile is preferably tangent or nearly tangent to the cable's outside diameter. Thus, tangent wall 32 is ideal for neck region 22.
The solidified potted region expands as one proceeds from the anchor's neck region toward the distal region. A relatively rapid expansion can be used to form a “shoulder” in the wall profile. FIG. 5 shows a shoulder 34 formed by a relatively steeply sloping wall profile in mid region 24. This forms a solid mechanical interference which will hold the potted mass in place. The potted mass lying between the shoulder and the neck region is preferably allowed to elongate (“seat”) somewhat under tension, thereby forming a more even stress distribution. Thus, the inclusion of a shoulder is preferable for the mid region.
Of course, if one continues the steeply sloping wall profile of the shoulder toward the anchor's distal end, the anchor will have to be made very large to contain the profile. The stress tends to diminish as one approaches the distal region. Thus, there is little to be gained by continuing the steeply sloping profile of the shoulder. At some point it is preferable to discontinue the sloping wall profile and employ a profile having a more moderate slope. FIG. 5 shows the use of such a portion, which is designated as extension wall 36.
The reader will thereby perceive the differing and somewhat contradictory design goals present in the anchor's neck, mid, and distal regions. Several prior art anchors have attempted to reconcile these conflicting goals. FIG. 6 is a sectioned elevation view of one such prior art anchor. The wall profile is a revolved constant radius arc 38 (revolved around central axis 51). Arc center 40 is positioned so that tangency point 74 is created with the cable at the point where the cable exits the anchor. Thus, the goal of creating tangency with the cable is met.
The goal of creating a shoulder in the mid region can also be met using a constant radius arc. The reader will observe in the example illustrated that the wall profile has a fairly steep slope in the mid region, thereby forming a suitable shoulder 34. The problem with the use of the constant radius arc in this fashion is the slope existing between tangency point 74 and the shoulder. The wall's slope increases fairly rapidly as one proceeds from tangency point 74 toward the distal anchor boundary. A more gradually increasing slope is preferable, since this would allow the potted mass in the vicinity of the neck to elongate somewhat under tension. This elongation produces a more even stress distribution. However, the rapidly increasing slope inherent in the constant radius arc design prevents the solidified potted region in the vicinity of the neck from elongating without experiencing excessive compressive stress. Thus, the use of the constant radius arc tends to concentrate stress in the neck region. The result is an anchor which fails below the ultimate tensile strength of the cable itself.
FIG. 7 shows another prior art geometry which attempts to address the problem of stress concentration in the neck region. In the anchor illustrated in FIG. 7, the revolved wall is defined by a portion of a parabola 42. The parabola's focus 44 is positioned appropriately—and the constants governing the parabola are appropriately selected—to produce a wall profile such as shown. Parabolic wall 45 includes a shoulder 34 in the mid region. It also includes a slope in the neck region which is not rapidly changing (and therefore produces a reasonably even stress distribution in the neck region). However, the reader will observe the presence of non-tangent condition 46 at the neck anchor boundary. This non-tangent condition produces a significant stress concentration at the point where the cable exits the neck anchor boundary.
Those skilled in the art will readily appreciate that one way to create a tangent condition at the neck anchor boundary using a parabola is to make the outside diameter of the cable an asymptote of the parabola. Unfortunately, making the outside diameter of the cable an asymptote will mean that the parabolic wall profile will have insufficient slope to form the necessary mechanical interference. This explains why anchors using parabolic wall profiles have been forced to use a non-tangent condition at the neck anchor boundary. The result is an undesirable stress concentration in the neck region. Like the version using the constant radius arc, the termination of FIG. 7 tends to fail well short of the cable's ultimate tensile strength.
An ideal wall geometry will include a tangent condition at the neck anchor boundary, a shoulder in the mid region, and an appropriate stress distributing transition in the wall slope therebetween. The present invention achieves these goals, as will be explained.